Field of the Invention
The invention relates to an intake valve for an internal combustion engine with direct fuel injection.
Intake and outlet valves are precision engine components for blocking cross sections of flow used for controlling the gas exchange in internal combustion engines. Such valves are expected to seal the working chamber of the cylinder from the outside. Intake valves are thermally stressed to a lower degree than outlet valves because they are cooled by the flow of fresh gas streaming around these valves. It is therefore easier to control the problems associated with intake valves than those attendant to outlet valves. However, as part of the tendency to save energy, design engineers of internal combustion engines are constantly faced with the challenge of meeting higher fuel saving requirements. These engineers are expected to come up with more favorable fuel consumption values.
Injecting the fuel directly into the combustion chamber offers more favorable fuel consumption values as associated with Otto engines versus an external formation of the mixture, which is the most widely used method at the present time. However, direct fuel injection for Otto engines is not entirely free of problems. Although intake valves are cooled by the combustion air flowing in, deposits show after relatively short motor running time periods in the region of the valve shaft, which is freely standing in the intake channel. These deposits continue to grow during use to such an extent that any further operation of the engine is no longer possible without cleaning the valve in the region of the hollow throat and/or the constriction.
Therefore, this invention protects the zone of the hollow throat against deposits up to the stripping edge in the region of the transition from the valve shaft to the valve disk. The often-observed mechanism of the formation of deposits in these zones has to be suppressed or at least substantially reduced. This zone of the valve is known to be heated by heat conduction, starting from the surface of the disk facing the combustion chamber, and commonly reaches temperatures leading to deposits of oil residues or combustion residues. Oil residues are caused by the passage of oil on the valve control. Combustion residues may deposit in the area of the hollow throat by a return of exhaust gas. In a vehicle with direct fuel injection, the deposits require cleaning of the valve after a running time of the engine corresponding with a driven distance of about 60,000 km. The valves have to be removed from the engine for cleaning.
It has been found that a critical temperature is responsible for the formation of deposits, which are primarily caused by the carbonization (or coking) of oil residues. This critical temperature range reaches from about 80xc2x0 C. to 380xc2x0 C. Coatings of deposits do not adhere below this range. Above 380xc2x0 C., the depositing oil carbon is burned (incinerated) and scales off. It is known, furthermore, that if the mixture is formed outside of the combustion chamber, there are distinctly fewer problems with deposits.
Therefore, a valve naturally has to have a temperature that is lower than 180xc2x0 C. or higher than 380xc2x0 C. Reducing the temperature could be accomplished with materials exhibiting superior thermal conductivity, or by interrupting the conduction of heat in the valve, for example by means of an unfilled cavity.
A valve seat ring with poor thermal conductivity or a filled hollow valve assuring superior heat transport from the disk in the direction of the shaft could lead to a temperature higher than 380xc2x0 C. However, these technical solutions, which are entirely realizable, result in much higher costs.
A valve for internal combustion engines, in particular for engines with fuel injection, which has a protective jacket, is already known from DE-OS 33 33 326. The protective jacket as opposed to the valve is designed as a structural element that is capable of oscillating, so that deposits of the fuel admixed to the combustion air are avoided by knocking them off. Experience shows that this solution has not been successful with vehicle engines with direct injection of gasoline. The valves have to be cleaned after a driven distance of about 60,000 km.
Furthermore, the use of a screening element is known from DE-PS 814 682. In particular, with outlet valves, this screening element is expected to protect the backside of the valve against excessive introduction of heat by the combustion gases flowing by. The problems known regarding outlet valves caused by high temperatures, however, do not arise with intake valves because a flow of cool fresh gases is cooling these intake valves.
One object of the invention is to provide an intake valve made from a material used for an internal combustion machine with direct fuel injection whose surface comes into contact with the combustion air flowing in. This intake valve is protected against deposits in the region between the hollow throat and the stripping edge on the valve shaft by simple means at favorable cost. This problem is solved by the screening element in providing a transition zone extending from the zone of the hollow throat of the valve disk up to the stripping edge of the valve shaft. This screening element has a separate contact or guide surface for the combustion air fed into the combustion chamber and has a surface temperature lower than the carbonization (or coking) temperature of the motor oil.
A suitable screening element could be a thin-walled sleeve enclosing the construction of the wall while maintaining a defined intermediate space.
The sleeve is preferably retained in its seat by friction grip. In another advantageous development of the invention, the end of the sleeve facing the valve shaft has cuts or slits pointing in the longitudinal direction for forming elastically bending lamellas.
In another advantageous development of the invention, the sleeve is manufactured from a stainless steel that can be deep-drawn, so that it can be formed in a simple manner as a conical shape. A tubular material can be advantageously used if the sleeve needs to have a cylindrical form.
Furthermore, it is possible to coat the surface of the screening element with a material which reduces adhesion.
To obtain, a flat abutment on the valve when a cylindrical sleeve is used, the ends of the lamellas can be deformed so that a defined intermediate space is maintained between the sleeve and the surface of the valve at the same time.
Engine tests with the intake valves of the invention have also shown the formation of deposits in the region of the hollow throat but after a longer time of operation of the engine as well. Because the sleeve is mounted as a screening element, a separation is achieved between the hot surface of the valve and the atmosphere surrounding the valve. The temperature of the surface of the sleeve is decisively below the temperature of the surface of the valve, so that the surface of the sleeve exposed to the combustion air flowing in around it showed either no deposits or substantially reduced rates of residue deposits.